This invention relates to gradient systems for liquid chromatography.
Gradient programmers are known which control the flow of two or more solvents to a mixer to provide a constant flow rate of influent to a chromatographic column from the mixer while varying the proportions of the two or more solvents.
In one prior art programmer of this type, two pumps pump different solvents into a mixer, which mixes them into a final influent to the chromatographic column. The rate of pumping of the two pumps is varied with time so that their sum is constant but the proportions of the solvent supplied by each differ.
This type of prior art gradient former has several disadvantages such as: (1) when one of the pumps is pumping at a very low rate or when both are pumping at nearly the same but differing rates, substantial inaccuracies occur caused by pulsations; and (2) in a high pressure liquid chromatograph, the two high pressure pumps increase the cost of the system.
In another prior art system, three low pressure pumps directly feed one high pressure pump. Such a system is disclosed in U.S. Pat. No. 4,311,586. This system has a disadvantage of being expensive.
In still another prior art system, digitally controlled valves are controlled in response to a computer command and each supplies a solvent to a chromatographic pump from a different reservoir. A system of this type is disclosed in U.S. Pat. No. 4,128,476 issued Feb. 2, 1982, to John V. Rock. This system has a disadvantage of risking overlapping valve openings and thus imprecise compositions of liquids.